1. Field of the Invention
This invention relates to a structure of a hole in differential wiring in a multi-layer wiring board.
2. Description of the Related Art
FIG. 8 illustrates an example of a connection by a through-hole for connecting wiring patterns 6 and 7 in the differential wiring and wiring patterns 8 and 9 in the differential wiring in the multi-layer wiring board according to the related art.
FIG. 9 shows a cross-sectional view of the multi-layer wiring board in a direction of a thickness of the multi-layer wiring board. FIG. 10 shows a cross-sectional view along IIIxe2x80x94III line in FIG. 9.
The multi-layer wiring board includes the wiring patterns 6 and 7 and insulating layers 3, 4 and 5 formed to sandwich electric power or ground layers 1 and 2. The wiring patterns 6 and 7 are facing the insulating layer 3 and the electric power or ground layer 1. The wiring patterns 6 and 7 constitute a differential micro strip line or strip line. The wiring patterns 8 and 9 are facing the insulating layer 5 and the electric power or ground layer 2. The wiring patterns 8 and 9 constitute the differential micro strip line or strip line. For connecting the wiring pattern 6 and the wiring pattern 8 which are different signal layers, a hole is formed through the multi-layer wiring board at first, and a circumference of the hole is plated with a conducting material to form a through-hole 19. A through-hole 20 connecting the wiring pattern 7 and the wiring pattern 9 is formed in the same way.
The differential wiring is realized through a following procedure. At first, when a signal is transmitted to a wiring pattern (called as xe2x80x9ca first wiring patternxe2x80x9d), a complete reverse phase signal is transmitted to an adjacent wiring pattern (called as xe2x80x9ca second wiring patternxe2x80x9d). Then, the signal transmitted to the first wiring pattern and the signal transmitted to the second wiring pattern are coupled in an electromagnetic field. Two wiring patterns become a pair and transmit the signals as one signal. As stated, the differential wiring is used for realizing a transmission method which has less influence of an external noise. Since phases of two wiring patterns in the differential wiring are constantly reverse each other, one of two wiring patterns is called as positive (+) side and another one is called as negative (xe2x88x92) side. When a high speed signal is transmitted to the wiring patterns of differential pair, an impedance and a coupling in the differential wiring are controlled by selecting an appropriate pattern width, pattern interval, thickness of the layers and thickness of an insulating layer in the multi-layer wiring board, and dielectric constant of an insulator. Accordingly, the signal are transmitted efficiently.
For most of through-holes in the multi-layer wiring boards according to the related art, the impedance was not considered at all. Japanese Unexamined Published Patent Application Hei 6-37416 discloses a through-hole in which the impedance is considered. FIG. 11 illustrates an example. In FIG. 11, a through-hole 31 includes a first copper plating portion 32 for a signal, located at a center, a second copper plating portion 34 including a ground potential, surrounding the first copper plating portion 32, and an insulating portion 33 filling a space between the first copper plating portion 32 and the second copper plating portion 34. The through-hole 31 constitutes a coaxial cable structure. In this way, the impedance is controlled.
However, in the through-hole structure according to the related art, for the wiring patterns in the differential wiring, a differential pair is not coupled. Therefore, a tolerance to the external noise, which is the characteristics of a differential signal, becomes lower.
It is one of objects of this invention to structure a transmission line system in which a reflection is suppressed by preventing an anomaly in the impedance between the wiring patterns in the differential wiring and the hole.
It is another object of this invention to increase the tolerance to the external noise by structuring the hole by a pair in the differential wiring, coupling the holes in the electromagnetic field and selecting a length (distance, width) between the holes coupled as the pair in the differential wiring and a dielectric constant of an insulating resin appropriately so that the differential pair is coupled firmly.
According to an aspect of this invention, a wiring board includes a base board, a first hole, formed through the base board, including an insulating portion filled with an insulator and a pair of second holes formed within the first hole through the insulating portion.
According to another aspect of this invention, a method for manufacturing a wiring board includes forming a first hole through a base board, forming an insulating portion by filling the first hole with an insulator and forming a pair of second holes through the insulating portion.
According to another aspect of this invention, a method for manufacturing the wiring board includes forming a first hole through a base board including conductor layers at an upper surface and a lower surface, forming an insulating portion by filling the first hole with an insulator, forming insulating layers on both the upper surface and the lower surface of the base board by using the insulator and forming a pair of second holes through the insulating portion and the insulating layers.
Further features and applications of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Other objects features, and advantages of the invention will be apparent from the following description when taken in conjunction with the accompany drawings.